Question

A plano convex lens has diameter of 10 cm and its thickness at the centre is 0.5 cm. Speed of light in the lens is 2 × 10⁸ ms^-1. What is the focal length of the lens?

विकल्प

• 10 cm

• 17.5 cm

• 10.5 cm

• 21 cm

Answer 1

10.5 cm

Explanation:

Consider

∴ (R – 0.5)² = R² – 5²

R² – R + 0.25 = R² – 5

R = 5.25 cm

∴ `1/f = (mu-1)/R`

= `(((3xx10^8)/(2xx10^8)-1)/5.25)`

= `1/10.5`

f = 10.5 cm